docs: def _get_plugin_install_dirs():
      '''Return all the places on the filesystem where we should look for plugin
      definitions. Order is significant here: user-installed plugins should be
      searched first, followed by system-installed plugins, and last of all peru
      builtins.'''
      builtins_dir = os.path.join(compat.MODULE_ROOT, 'resources', 'plugins')
      if os.name == 'nt':
          # Windows
          local_data_dir = os.path.expandvars('%LOCALAPPDATA%')
          program_files_dir = os.path.expandvars('%PROGRAMFILES%')
          return (
              os.path.join(local_data_dir, 'peru', 'plugins'),
              os.path.join(program_files_dir, 'peru', 'plugins'),
              builtins_dir,
          )
      else:
          # non-Windows
          default_config_dir = os.path.expanduser('~/.config')
          config_dir = os.environ.get('XDG_CONFIG_HOME', default_config_dir)
          return (
              os.path.join(config_dir, 'peru', 'plugins'),
              '/usr/local/lib/peru/plugins',
              '/usr/lib/peru/plugins',
              builtins_dir,
          )